Syntax

[ #if expression | #ifdef identifier | #ifndef identifier ]
{ statement }...
{#elif expression
{ statement }... }...
{#else
{ statement }... }
#endif

Notes

#if	Used when evaluating an expression.
#ifdef	Used to check if an identifier is defined.
#ifndef	Used to check if an identifier is not defined.
#elif	Used when evaluating an expression. Only evaluated when the preceding if or elif directives evaluate to false.
#else	Used only used when the preceding #if, #ifdef, #ifndef or #elif directives evaluate to false.
#endif	Used to end #if, #ifdef and #ifndef directives.
expression	the expression to evaluate.
identifier	the identifier to check for existance.
statement	the statement(s) to compile when the preceding #if, #ifdef, #ifndef or #elif evaluates to true.

Example

#ifndef EOF
    #define EOF (-1)
#endif

#if VERSION == 1
    #include <v1sup.h>
#elif VERSION == 2
    #include <v2sup.h>
#elif VERSION == 3
    #include <v3sup.h>
#else
    #error version not supported
#endif